I would like to hear about your favourite RPGs (plot and mechanics in particular) as well
At a cursory look, a broad list of what I consider S-Class would be Dark Sun: The Shattered Lands, Fallout (Original), The Temple of Elemental Evil, Geneforge (any) Arcanum, Neverwinter Nights: Hordes of the Underdark, Neverwinter Nights II: Mask of the Betrayer, Elder Scrolls III: Morrowind, Fallout: New Vegas, Planescape: Torment, Kotor II, System Shock 2, Deus Ex (Original), Mass Effect (Original only, heavy emphasis on the only), Prey (2017), Shadowrun: Dragonfall, Expeditions: Conquistador, any FromSoftware Soulsbourne, Persona (any), FFVI, the GBA-era Fire Emblems, Fire Emblem: Three Houses. I might add more or remove some of these after further reflection. If Zelda counts as an RPG I would throw in Majora's Mask and A Link to the Past for sure.
The A-Class would be far more expansive and would include both Owlcat RPGs. Kingmaker could almost have been S-class for me but simply took too long to finish. I also dislike real-time with pause for the most part, that didn't help.